Add PLACEHOLDER_ISSUANCE_VALUE to simplicity-core

[ikripaka]

Add a constant, as the value field is optional for issuance tokens.
The actual value is irrelevant for further transaction analysis and can be omitted.

[KyrylR]

Could you explain more?

Currently it looks like a Magic value

[ikripaka]

The idea is to use fixed value instead of any value, which is derived from utxos.
Yeah, in some kind this value is used for blinding, but it also can be executed with PLACEHOLDER_ISSUANCE_VALUE.
In further code on transaction analysis value field is unused by now, so I wished to replace it with constant.

options/creation_option.rs basic/issue_asset.rs

[KyrylR]

In comment for this constant we should mention this example, and add reasoning why it can be done, and mention when the actual values should be in TxOutSecrets

[ikripaka]

The main information is in the comment. The reissuance and issuance tokens have to be defined and used in the order they were created, not the other way around.
https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/blind.rs#L652

How the blinding happens:

1. https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/pset/mod.rs#L479
   Blind last - the function blinds all inputs and commitments to the last output.
2. https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/pset/mod.rs#L493
    Inside this function, we can see that first, we gather SurjectionProofs and then blind the outputs.
     Gathering surjection inputs has a condition related to the issuance token. We can see that only asset_id is copied and the value is set to 0.
   // This value really does not matter in surjection proofs
     https://github.com/ElementsProject/rust-elements/blob/2d94f5dcb5e4a81838681c71ed8a49b337feecd2/src/pset/mod.rs#L438

So, after that, we can say with confidence that the value can be omitted and set to PLACEHOLDER_ISSUANCE_VALUE.

[KyrylR]

ACK 1b51de6